Goshogake and Tamagawa Hot Springs
Formation process of mud structures
In Goshogake, geothermal manifestations are mud volcanoes, mud pots, hot pools, and blowing springs (water gushing out with gas). Their formation mechanism is thought as follows (Miyagi and Saito, 1983). Interaction of vapor including hydrogen sulfide (H2S) with ground water precipitates sulfur and iron sulfide. The deposits mix with altered rock and then form mud pods (early type) by spouting with vapor on the surface. Furthermore, the mud pods of early type develop into mud volcanoes, mud pods, or hot pools depending on pressure of vapor and the amount of ground water and deposits. Hot pools are formed when deposits have considerable water and mud volcanoes when deposits have less water. Blowing springs are made in the case where vapor contacts with surface water. However, unlike this model, some mud volcanoes and mud pots are formed in hot pools like Odoro Kazan. This may be because deposits fill the hot pool and supplied water decreases following the formation of hot pools.
Fig. 2: Formation process of mud structures (Miyagi and Saito, 1983)
Generally, water is transparent without mud in hot springs where a large amount of water is discharged. In the Tamagawa hot springs mentioned below, there is neither mud volcanoes nor mud pots. The discharge of hot water in a single spring in Tamagawa is 9000 kg a MINUTE, although that in Oyunuma is 9000 kg a HOUR.
Tamagawa hot springs
Photo 4: Tamagawa hot springs 
The Tamagawa hot spring area where strong sulfurous odor is hanging in the air is much more lively than Goshogake. The springs are volcanic acid hot springs welling up in the explosion crater of Yakeyama. Visitors can observe the Obuki spring spouting out the largest volume of the strongest acid hot water in Japan, “yubatake” where sinter is collected in a stream, and fumaroles ejecting volcanic gas.
Photo 5: Obuki spring 
Photo 5 shows the Obuki spring, one of the Tamagawa hot springs. This extraordinary hot spring has been attracted attention; scientists are interested in its geological and chemical characteristics and ordinary people in the efficacy for health as bathing. The discharge volume of geothermal water is about 9000 L/min, the largest volume of water gushing from a single spring in Japan. The water has a temperature of 97°C and pH 1.2 (the lowest value among Japanese hot springs), containing a large quantity of hydrochloric acid (HCl). The properties of geothermal water of Tamagawa hot springs will be explained later.
Yubatake, collecting sinter
Various chemical compositions are contained in geothermal water. When the water flows out at a hot spring, they precipitate due to changes of physical and chemical conditions such as decrease in temperature and pressure, and variation of pH. Hot spring deposits include sulphurous sinter, calcareous sinter, siliceous sinter, iron sinter (limonite), and sulfate sinter.
The Obuki spring is the headwaters of stream, Yukawa. There is equipment like a flume or gutter for collecting sinter in Yukawa (Photo 6). The compositions of the hot spring water are deposited from the water running through the equipment. Such field is often called “Yubatake”. The deposits of the Obuki spring water consist mainly of sulfur (main component), tridymite and cristobalite (SiO2), and barite (BaSO4). The stream bed is yellow, yellowish green, or red due to deposits. In Japan, sinter is used for a bath at home and medicine, often sold as a souvenir.
Hokutolite [(Ba, Pb)SO4] is a rare mineral of which barium sulfate (barite) includes lead sulfate (anglesite). This mineral is yielded only in the Tamagawa hot springs in Japan, designated as a Special National Treasure. It is also known to be collected in Peitou Hot Spring in Taiwan. Hokutolite is deposited from the Obuki spring water but the locality is not disclosed to prevent the theft of that by illegal digging.
Fig. 3: Crystal structure
Barite is composed of barium and sulfate ions arranged alternately in a three-dimensional framework. Some of the barium ions (Ba2+) are substituted for lead ions (Pb2+). Hokutolite also includes a trace of radium (Ra2+), strontium (Sr2+), calcium (Ca2+), and ferrous (Fe2+) ions. The ratios of lead and radium ions to barium ions are 1:5 to 20, 1:1 billion, respectively. Hokutolite has radioactivity because of a radioelement, radium. The mineral is produced by deposition of components in hot spring water, the growth rate of which 1 mm per 10 years.
A photo of Hokutolite is found here on the Kyushu University Museum website. This Hokutolite occurred in Taiwan but the appearance is similar to a Hokutolite specimen that is exhibited in Mining Museum of Akita University, collected in the Tamagawa hot spring. The appearance has a striped pattern because brown or white crystals grow depending on the amount of ions including Fe2+.